Method for desulfurizing gasoline or diesel fuel for use in a fuel cell power plant

ABSTRACT

A fuel processing method is operable to remove substantially all of the sulfur present in an undiluted oxygenated hydrocarbon fuel stock supply which contains an oxygenate and which is used to power a fuel cell power plant in a mobile environment, such as an automobile, bus, truck, boat, or the like, or in a stationary environment. The power plant hydrogen fuel source can be gasoline, diesel fuel, or other like fuels which contain relatively high levels of organic sulfur compounds such as mercaptans, sulfides, disulfides, and the like. The undiluted hydrocarbon fuel supply is passed through a desulfurizer bed wherein essentially all of the sulfur in the organic sulfur compounds reacts with the nickel reactant, and is converted to nickel sulfide, while the now desulfurized hydrocarbon fuel supply continues through the remainder of the fuel processing system. The method does not require the addition of steam or a hydrogen source to the fuel stream prior to the desulfurizing step. The method can be used to desulfurize either a liquid or a gaseous fuel stream, which contains an oxygenate such as MTBE, ethanol, methanol, or the like. The inclusion of the oxygenate serves to extend the useful life of the desulfurization method.

TECHNICAL FIELD

[0001] The present invention relates to a method for desulfurizinggasoline, diesel fuel or like hydrocarbon fuel streams so as to renderthe fuel more suitable for use in a mobile vehicular fuel cell powerplant assembly. More particularly, the desulfurizing method of thisinvention is operable to remove organic sulfur compounds found ingasoline to levels which will not poison the catalysts in the fuelprocessing section of the fuel cell power plant assembly. The method ofthis invention involves the use of a nickel reactant bed which has anextended useful life cycle due to the inclusion of oxygenates in thefuel stream in appropriate amounts.

BACKGROUND OF THE INVENTION

[0002] Gasoline, diesel fuel, and like hydrocarbon fuels have not beenuseful as a process fuel source suitable for conversion to a hydrogenrich stream for small mobile fuel cell power plants due to the existenceof relatively high levels of naturally-occurring complex organic sulfurcompounds. Hydrogen generation in the presence of sulfur results in apoisoning effect on all of the catalysts used in the hydrogen generationsystem in a fuel cell power plant. Conventional fuel processing systemsused with stationary fuel cell power plants include a thermal steamreformer, such as that described in U.S. Pat. No. 5,516,344. In such afuel processing system, sulfur is removed by conventionalhydrodesulfurization techniques which typically rely on a certain levelof recycle as a source of hydrogen for the process. The recycle hydrogencombines with the organic sulfur compounds to form hydrogen sulfidewithin a catalytic bed. The hydrogen sulfide is then removed using azinc oxide bed to form zinc sulfide. The general hydrodesulfurizationprocess is disclosed in detail in U.S. Pat. No. 5,292,428. While thissystem is effective for use in large stationary applications, it doesnot readily lend itself to mobile transportation applications because ofsystem size, cost and complexity. Additionally, the gas being treatedmust use process recycle in order to provide hydrogen in the gas stream,as noted above.

[0003] Other fuel processing systems, such as a conventional autothermalreformer, which use a higher operating temperature than conventionalthermal steam reformers, can produce a hydrogen-rich gas in the presenceof the foresaid complex organic sulfur compounds without priordesulfurization. When using an autothermal reformer to process raw fuelswhich contain complex organic sulfur compounds, the result is a loss ofautothermal reformer catalyst effectiveness and the requirement ofreformer temperatures that are 200° F.-500° F. higher than are requiredwith a fuel having less than 0.05 ppm sulfur. Additionally, a decreasein useful catalyst life of the remainder of the fuel processing systemoccurs with the higher sulfur content fuels. The organic sulfurcompounds are converted to hydrogen sulfide as part of the reformingprocess. The hydrogen sulfide can then be removed using a solidabsorbent scrubber, such as an iron or zinc oxide bed to form iron orzinc sulfide. The aforesaid solid scrubber systems are limited, due tothermodynamic considerations, as to their ability to lower sulfurconcentrations to non-catalyst degrading levels in the fuel processingcomponents which are located downstream of the reformer, such as in theshift converter, or the like.

[0004] Alternatively, the hydrogen sulfide can be removed from the gasstream by passing the gas stream through a liquid scrubber, such assodium hydroxide, potassium hydroxide, or amines. Liquid scrubbers arelarge and heavy, and are therefore useful principally only in stationaryfuel cell power plants. From the aforesaid, it is apparent that currentmethods for dealing with the presence of complex organic sulfurcompounds in a raw fuel stream for use in a fuel cell power plantrequire increasing fuel processing system complexity, volume and weight,and are therefore not suitable for use in mobile transportation systems.

[0005] An article published in connection with the 21st Annual PowerSources Conference proceedings of May 16-18, 1967, pages 21-26, entitled“Sulfur Removal for Hydrocarbon-Air Systems”, and authored by H. J.Setzer et al, relates to the use of fuel cell power plants for a widevariety of military applications. The article describes the use of highnickel content hydrogenation nickel reactant to remove sulfur from amilitary fuel called JP-4, which is a jet engine fuel, and is similar tokerosene, so as to render the fuel useful as a hydrogen source for afuel cell power plant. The systems described in the article operate atrelatively high temperatures in the range of 600° F. to 700° F. Thearticle also indicates that the system tested was unable to desulfurizethe raw fuel alone, without the addition of water or hydrogen, due toreactor carbon plugging. The carbon plugging occurred because thetendency for carbon formation greatly increases in the temperature rangebetween about 550° F. and about 750° F. A system operating in the 600°F. to 700° F. range would be very susceptible to carbon plugging, as wasfound to be the case in the system described in the article. Theaddition of either hydrogen or steam reduces the carbon formationtendency by supporting the formation of gaseous carbon compounds therebylimiting carbon deposits which cause the plugging problem.

[0006] It would be highly desirable from an environmental standpoint tobe able to power electrically driven vehicles, such as an automobile,for example, by means of fuel cell-generated electricity; and to be ableto use a fuel such as gasoline, diesel fuel, naphtha, lighterhydrocarbon fuels such as butane, propane, natural gas, or like fuelstocks, as the fuel consumed by the vehicular fuel cell power plant inthe production of electricity. In order to provide such a vehicularpower source, the amount of sulfur in the processed fuel gas would haveto be reduced to and maintained at less than about 0.05 parts permillion.

[0007] The desulfurized processed fuel stream can be used to power afuel cell power plant in a mobile environment or as a fuel for aninternal combustion engine. The fuel being processed can be gasoline ordiesel fuel, or some other fuel which contains relatively high levels oforganic sulfur compounds such as thiophenes, mercaptans, sulfides,disulfides, and the like. The fuel stream is passed through a nickeldesulfurizer bed wherein essentially all of the sulfur in the organicsulfur compounds reacts with the nickel reactant and is converted tonickel sulfide leaving a desulfurized hydrocarbon fuel stream whichcontinues through the remainder of the fuel processing system.Previously filed U.S. patent applications Ser. No. 09/104,254, filedJun. 24, 1998; and Ser. No. 09/221,429, filed Dec. 28, 1998 describesystems for use in desulfurizing a gasoline or diesel fuel stream foruse in a mobile fuel cell vehicular power plant; and in an internalcombustion engine, respectively.

[0008] We have discovered that desulfurization of a gasoline or dieselfuel stream which uses a nickel catalytic adsorbant bed cannot beperformed over a significantly extended period of time unless the fuelstream includes an oxygenate compound in appropriate proportions.Various oxygenates could suffice for the desulfurization processincluding MTBE, ethanol or other alcohols, ethers, or the like.

DISCLOSURE OF THE INVENTION

[0009] This invention relates to an improved method for processing agasoline, diesel, or other hydrocarbon fuel stream over an extendedperiod of time, which method is operable to remove substantially all ofthe sulfur present in the fuel stream.

[0010] Gasoline, for example, is a hydrocarbon mixture of paraffins,napthenes, olefins and aromatics, whose olefinic content is between 1%and 15%, and aromatics between 20% and 40%, with total sulfur in therange of about 20 ppm to about 1,000 ppm. The national average for theUnited States is 350 ppm sulfur. The legally mandated average for theState of California is 30 ppm sulfur. As used in this application, thephrase “California Certified Gasoline” refers to a gasoline which hasbetween 30 and 40 ppm sulfur content, and which contains about 11% byvolume MTBE at the present time. California Certified Gasoline is usedby new car manufacturers to establish compliance with Californiaemissions certification requirements.

[0011] We have discovered that the presence of oxygenates in thegasoline, like MTBE (methyl-tertiary-butyl ether, i.e., (CH₃)₃COCH₃), orethanol, for example, prevent rapid deactivation of the nickel catalyticadsorption of organic sulfur compounds from the fuel stream. Ethanolcould be an appropriate solution to this problem since it is non-toxic,is not a carcinogen, and is relatively inexpensive and readily availablein large supplies as a byproduct of the agriculture industry. Methanol,which would also extend the desulfurizer bed life, is not preferredsince it is toxic; while MTBE is likewise not preferred since it isthought to be a carcinogenic compound, and may be banned in certainareas of the United States in the near future by new environmentalregulations. Preferred oxygenates are non-toxic and non-carcinogenicoxygen donor compounds, such as ethanol or the like.

[0012] The effectiveness of a nickel adsorbent reactant to adsorborganic sulfur compounds from gasoline depends on the relative coverageof the active reactant sites by adsorption of all the variousconstituents of gasoline. In other words, the catalytic desulfurizationprocess depends on the amount of competitive adsorption of the variousconstituents of gasoline. From the adsorption theory, it is known thatthe relative amount of adsorbate on an adsorbent surface dependsprimarily on the adsorption strength produced by attractive forcesbetween the adsorbate and adsorbent molecules; secondarily on theconcentration of the adsorbate in the gasoline, and temperature.Coverage of a reactant surface by an adsorbate increases with increasingattractive forces; higher fuel concentration; and lower temperatures.Relative to gasoline, Somorjai (introduction to Surface Chemistry andCatalysis, pp, 60-74) provides some relevant information on theadsorption of hydrocarbons on transition metal surfaces, such as nickel.Saturated hydrocarbons only physically adsorb onto the nickel reactantsurface at temperatures which are less than 100° F., thereforeparaffins, and most likely napthenes, won't compete with sulfurcompounds for adsorption sites on the nickel reactant at temperaturesabove 250° F. and 300°°F. On the other hand, unsaturated hydrocarbons,such as aromatics and olefins, adsorb largely irreversibly on transitionmetal surfaces even at room temperature. When an unsaturated hydrocarbonsuch as an aromatic or an olefin adsorbs on a transition metal surface,and the surface is heated, the adsorbed molecule rather than desorbingintact, decomposes to evolve hydrogen, leaving the surface covered bythe partially dehydrogenated fragment, i.,e., tar or coke precursors. Wehave discovered that, at 350° F., unsaturated hydrocarbons are nearlycompletely dehydrogenated, and the dehydrogenated tar fragments formmultiple carbon atom-to-nickel reactant surface bonds. This explains whyaromatics and olefins in gasoline, in the absence of oxygenatedcompounds in appropriate concentrations, will deactivate the nickelnickel reactant from adsorbing sulfur after a relatively short period oftime.

[0013] In general, the adsorption strength of a compound depends on thedipole moment, or polarity, of the molecule. A higher dipole momentindicates that the compound is more polar and is more likely to adsorbon a reactant surface. Aromatics are an exception to this rule becausetheir molecular structure includes a n ring of electron forces thatproduces a cloud of induced attractive forces with adjacent surfaces.Based on the dipole moments of hydrocarbons, allowing for the rr ring inaromatics, the order of adsorption strength (highest to lowest) is:nitrogenated hydrocarbons>oxygenatedhydrocarbons>aromatics>olefins>hydrocarbons containing sulfur>saturatedhydrocarbons. Since the adsorption strength of the oxygenatedhydrocarbons (such as ethanol, methanol, MTBE, or the like) is greaterthan that for aromatics and olefins, oxygenated hydrocarbons, or otheroxygen donor compounds, if present in the gasoline or diesel fuel streambeing desulfurized, will provide greater coverage of the nickel reactantsites than do the aromatics and olefins in the gasoline. Thus, theoxygenated hydrocarbons can reduce the adsorption of aromatics andolefins on the nickel reactant bed. Although saturated hydrocarbons(paraffins and cycloparaffins) would not be expected to be adsorbed onthe desulfurization nickel reactant to a significant extent, oxygenatedhydrocarbons will also prevent them from adsorbing onto the nickelreactant.

[0014] We have also discovered that the adsorbed oxygenated hydrocarbonsdo not inhibit the sulfur compounds from being adsorbed on the nickelreactant. The oxygenated hydrocarbons and the sulfur compounds are bothquite polar and therefore they are miscible, which allows the sulfurcompounds to dissolve into and diffuse through the adsorbed layer ofoxygenated hydrocarbon to the active nickel metal reactant sites. Thus,the oxygenated hydrocarbons provide a “shield” which inhibits thecarbon-forming hydrocarbons from contacting the nickel reactant siteswhile allowing the sulfur compounds to contact and react with the activenickel metal reactant sites.

[0015] Further non-essential but enabling information relating to thisinvention will become readily apparent to one skilled in the art fromthe following detailed description of a preferred embodiment of theinvention when taken in conjunction with the accompanying drawings inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a graph of the result of a short (seven hour)desulfurizer bed test run with three different modified formulations ofCalifornia Certified Gasoline showing the sulfur level in parts permillion (ppm) at the reactant bed exit for the various gasolineformulations, versus the test run operating time in hours;

[0017]FIG. 2 is a graph of the results of a longer desulfurizer bed testrun (about four hundred eighty five hours) with unmodified CaliforniaCertified Gasoline showing the sulfur level in the gasoline in ppm atthe nickel reactant bed exit, versus the operating time in hours;

[0018]FIG. 3 is a graph of the results of the same desulfurizer bed testrun shown in FIG. 2, but showing the oxygenate level in the gasoline, inpercent by weight, at the reactant bed exit, versus the test runoperating time in hours;

[0019]FIG. 4 is a graph of the result of a desulfurizer bed test runwith a commercially available gasoline showing the sulfur level in ppmat the nickel reactant bed exit versus the operating time of thedesulfurizer in hours;

[0020]FIG. 5 is a graph of the result of several different durationdesulfurizer bed test runs using different modified formulations ofCalifornia Certified Gasoline, one with, and one without oxygenates, andshowing the carbon level (in percent by weight) which was deposited onthe reactant in each successive section of the desulfurizer at the endof the test runs;

[0021]FIG. 6 is a graph of the sulfur content of the exit stream of adesulfurized gasoline fuel stream over a period of time at variedoperating temperatures, when a small amount of water is present, andwhen no water is present, in the fuel stream; and

[0022]FIG. 7 is a graph of the operating temperatures of the systemdescribed in FIG. 6 over the same period of time; and

[0023]FIG. 8 is a schematic view of an embodiment of the gasolinedesulfurizer system of this invention, which desulfurizes gasoline onboard a vehicle powered by a fuel cell power plant.

SPECIFIC MODES FOR CARRYING OUT THE INVENTION

[0024] Referring now to the drawings, FIG. 1 is a graph of the resultsof relatively short desulfurizer test runs using various formulations ofCalifornia Certified Gasoline, which graph shows the sulfur level in ppmfor the various formulations at the reactant bed exit, versus theoperating time of the test runs in hours. In these short term (sevenhour) test runs, sulfur was added to all of the California CertifiedGasoline formulations, so that the gasoline contained 240 ppm of sulfur.One of the gasoline formulations contained 11% MTBE by volume, which isan oxygenate and which is presently a conventional component ofCalifornia Certified Gasoline; another of the formulations contained 10%ethanol by volume, which is also an oxygenate; and the third formulationcontained essentially no oxygenate. In each of the test runs, thegasoline was run through a nickel reactant bed so as to attempt toremove sulfur from the gasoline. The trace line A shows the sulfurcontent of the gasoline formulation which did not contain an oxygenate.The sulfur content was measured at the exit end of the desulfurizerreactant bed. Trace A clearly shows that the oxygenate-free gasolineformulation had a steadily rising sulfur content at the desulfurizerexit during the duration of the test despite being run through thedesulfurizer reactant bed indicating deactivation of the desulfurizationreactant. Trace B shows the sulfur content of the gasoline formulationwhich contained MTBE. Trace C shows the sulfur content of the gasolineformulation which contained ethanol. This graph shows a majorimprovement and a decrease in sulfur at the reactant bed exit, when anMTBE or ethanol oxygenate is contained in the gasoline. This graph showsthat the oxygenate component of the gasoline prolongs the ability of thereactant bed to remove sulfur from the gasoline.

[0025]FIG. 2 is a graph of the results of a longer desulfurizer test runusing California Certified Gasoline which contained about 30 ppm sulfurand about 11% MTBE by volume. The test was run until sulfur breakthroughoccurred. The goal of the desulfurizer is to maintain the sulfur contentof the gasoline below about 0.05 ppm so that the gasoline will besuitable for processing for use in a mobile fuel cell power plant.Therefore, “sulfur breakthrough” is defined by our requirements asoccurring when a sustained post-reactant bed sulfur content of greaterthan about 0.05 ppm in the gasoline is present. The trace D shows thesulfur level in ppm at the exit of the reactant bed versus the operatingtime in hours and shows that the desulfurizer operated successfully forabout 400 hours with consistent sulfur levels in the nickel reactant bedexit stream of below 0.05 ppm. In this test run, the long term benefitof using an oxygenate in the fuel to minimize sulfur penetration throughthe desulfurizer device is demonstrated.

[0026]FIG. 3 is a graph of the results of the same longer termdesulfurizer test run shown in FIG. 2, but showing the oxygenate levelby percent weight at the nickel reactant bed exit versus the operatingtime in hours. From this figure, it will be noted that when the nickelreactant bed can no longer decompose the oxygenate, the nickel reactantloses its ability to remove organic sulfur compounds. It is noted fromtrace E in FIG. 3 that at about 400 hours, the MTBE content of the gasstream exiting the nickel reactant bed was about 11% by volume which isthe same concentration of MTBE in the gasoline stream entering thenickel reactant bed. Note that early in the test run, the nickelreactant bed is more capable of decomposing the MTBE, but this abilitygradually declines as the test run continues. This inability todecompose the oxygenate results in an increase in the sulfur content atthe nickel reactant bed exit, as shown in FIG. 2.

[0027]FIG. 4 is a graph of the results of another longer termdesulfurizer test run using a gasoline which had about a 90 ppm sulfurcontent and which contained about 11% MTBE by volume. Trace F shows thatthe sulfur level at the nickel reactant bed exit remained below 0.05 ppmfor about 125-135 hours, after which sulfur breakthrough occurred. Inthis test run, the long term benefit of using oxygenates in the fuel tominimize sulfur getting through the desulfurizing bed is alsodemonstrated.

[0028]FIG. 5 is a graph showing the results of two desulfurizer testruns using two different formulations of California Certified Gasoline,one containing an oxygenate (MTBE, 11% by volume), and the othercontaining no oxygenate. This graph shows the carbon level by percentweight deposited in each successive section of the desulfurizer nickelreactant bed. In this figure, the post test carbon content forsuccessive sections of the desulfurizers was measured and is shown fortwo tests that were run for different time periods, both of which wererun until sulfur breakthrough occurred. Trace H shows the results of thetest run for the gasoline formulation that contained no oxygenate. Thistest was run for 60 hours at which point in time, sulfur breakthroughoccurred. Trace G shows the results of the test run for the gasolineformulation that contained MTBE. This test was run for 485 hours atwhich point in time, sulfur breakthrough occurred. It was noted that thepresence or absence of the oxygenate in the gasoline being processed didnot effect the carbon build up profile on the nickel reactant bed, butit did increase the time period which is needed to reach the sulfurbreakthrough point in terms of carbon deposition. In each test, thedegree of carbon build up on the nickel reactant at the sulfurbreakthrough point in each section of the desulfurizer is almost exactlythe same. This figure demonstrates that “sulfur breakthrough” is afunction of the extent of carbon deposition on the nickel reactant bed,and is not a function of the extent of sulfur removal by the nickelreactant bed. This figure also demonstrates that the addition ofoxygenates to the gasoline retards carbon deposition on the nickelreactant bed, and thus enables extended sulfur removal from the fuelstream by the nickel reactant bed.

[0029] At this stage, we conclude that the presence of oxygenates in thegasoline maintains the desulfurization activity of the nickel reactantby significantly suppressing the carbon content (coke deposits andstrongly adsorbed species), and by keeping the nickel reactant activesites clean and available for desulfurization of the S-containingorganic molecules. As was mentioned before, this could be achieved by anin situ formation of hydrogen and/or water vapor due to the MTBEdecomposition process (chemical reaction effect). Therefore, we proposethat MTBE, and for the same reason any oxygenated organic molecule, isstrongly adsorbed on the nickel surface due to its high dipole momentwhere it decomposes to isobutylene and methanol. The adsorbed oxygenatedecomposes because the nickel reactant is very active and the C—O bondcan easily break. In general, the order in the required energy to breaka C—X bond is:

C—O<C—S<C—N<C—C<C—H

[0030] A nickel catalyst promotes the formation of methanol, a byproductof MTBE decomposition, or ethanol disproportionation reaction. Whenmethanol is decomposed, the following reactions occur:

4CH₃OH→3CH₄+CO₂+2H₂O  (1)

4CH₃OH→2CH₄+2CO₂+4H₂  (2)

[0031] For ethanol, the same reactions should produce ethane instead ofmethane. The presence of water vapor or hydrogen is well known tosuppress carbon formation, especially at elevated temperatures. Thehydrogen produced on the nickel reactant bed by equation (2) willhydrogenate carbon precursors emanating from the desulfurized organicsulfur components, and from the adsorbed/decomposed olefins andaromatics in the gasoline, through reaction with hydrogen emanating fromthe desulfurized fuel gas (Ely-Rideal mechanism) or through hydrogenspill over. Hydrogenation of carbon precursors from sulfur compounds,olefins and aromatics could occur entirely on the nickel reactantsurfaces from spill over of hydrogen generated by decomposition of theMTBE without initiating hydrogen exchange with the fuel gas stream.“Spill over” is the surface migration of hydrogen atoms from the nickelreactant site(s) that produce the hydrogen in equation (2) to thesite(s) that adsorb the olefins and aromatics.

[0032] The formation of hydrogen is demonstrated in Table 1 (below),which shows the decrease in olefin level during the desulfurizationprocess for the same commercially available gasoline containing MTBEshown in FIG. 4. Apparently, the hydrogen provided by decomposition ofMTBE serves to hydrogenate the olefins thereby forming saturatedparafins. It is apparent from Table 1 that the decomposition of MTBE notonly generates hydrogen, but also catalyzes the dehydrogenation ofnaphthenes to generate aromatics and more hydrogen.

[0033] Table 1 is a “PONA” (which is an acronym for paraffins, olefins,naphthene, and aromatics) analysis of the changes in PONA compoundswhich are found in the gasoline described in FIG. 4, both before andafter desulfurization; and also of the change in the sulfur content ofthe gasoline. TABLE 1 Hydrocarbon Type Before Desulfurization AfterDesulfurization Paraffins 38.8% 41.1% Olefins 14.9% 12.6% Naphthenes 9.6%  5.8% Aromatics 36.7% 40.6% Sulfur 90 ppm <0.05 ppm

[0034] Table 2 shows that, without MTBE, there is essentially no changein the “PONA” percentages in a low sulfur content, commerciallyavailable gasoline which is passed through the desulfurization nickelreactant bed. Also, Table 2 demonstrates that the sulfur content of thelow sulfur content gasoline still contains an unacceptably high contentof sulfur after the desulfurization step. TABLE 2 Hydrocarbon TypeBefore Desutfurization After Desulfurization Paraffins 64.6% 64.5%Olefins  3.7% 3.65% Naphthenes  2.89% 2.82% Aromatics 28.8% 29% Sulfur30.9 ppm 1.0 ppm

[0035] Desulfurization of a gasoline fuel sample containing about 30 ppmsulfur was carried out at a temperature of 375° F. FIG. 6 shows the exitstream desulfurization history of this low sulfur gasoline fuel sample.The desulfurization test run shown in FIG. 6 was run at a temperature of375° F., except for the time period between 73 and 120 hours. Duringthat time period, the reaction temperature was lowered to 350° F., asshown in FIG. 7. At the 375° F. operating temperature, the fuel streamexiting the desulfurizer nickel reactant bed contained about 1% to about2% water condensate which was derived from the MTBE. At the operatingtemperature of 350° F., the exiting fuel stream did not contain anyobvious water condensate. This fact confirms the formation of water, andcoextant superior desulfurization results obtained when water is presentin the fuel stream. It is noted from FIG. 6, that after the operatingtemperature is lowered to 350° F., and the water condensate in the fuelstream disappears, the sulfur level in the exiting fuel stream begins torise, and then, sometime after the operating temperature is increased,and the water condensate reappears in the fuel stream, the sulfur levelin the exiting fuel stream subsides.

[0036]FIG. 8 shows an embodiment of the desulfurization system of thisinvention wherein the desulfurizing bed 8 is positioned onboard avehicle 2. The system includes a fuel line 3 from the vehicle gas tankto a pump 4 which pumps the fuel through a line 6 to the desulfurizerbed 8. The bed 8 is heated to operating temperatures by an electricheater 10. The desulfurized gasoline passes from the desulfurizing bed 8through a line 12 to the fuel cell power plant 14 where the desulfurizedfuel is further processed and converted to electricity for powering thevehicle 2.

[0037] We have determined that the oxygenate not only protects thenickel reactant metal surface with an oxygenate “shield”, it alsoproduces hydrogen and water which enables the metal surface to remainfree of excessive carbon deposits for longer periods of time than if nooxygenate were present. The addition of very small quantities of waterin the fuel stream at the desulfurizer bed inlet, or the recirculationof a 1% to 10%, by volume, fraction of the fuel stream emanating from adownstream selective oxidizer outlet back to the desulfurizer bed inlet,would provide the same quantity of water and hydrogen as can be producedfrom the MTBE.

[0038] As a result, the MTBE could be eliminated from the gas streamwhen a fuel cell recycle stream is utilized. Minimal amounts of watercan be injected, either by itself, or when recycle is employed, contraryto the teachings of aforementioned Setzer et al article which waspublished in the 21 st Annual Power Sources conference proceedings,which article requires the use of three pounds of water for one pound offuel in order to reform the fuel gas stream.

[0039] By contrast, utilization of selective oxidizer exhaust willprovide only 2%-5% water for introduction into the desulfurizer bed,which would provide sufficient hydrogen to hydrogenate the adsorbedolefins and prevent the fouling of the metal nickel reactant surfacewith carbonaceous deposits. The operating range of 300° F.-450° F. forliquid fuels, and 250° F.-450° F. for gaseous fuels, both of which arebelow the temperature range suggested in the prior art for theperformance of a hydrodesulfurization process are available inperformance of this invention.

[0040] It will be readily appreciated that the addition of an effectiveamount of an oxygenate, or water, or a fuel cell fuel processing recyclestream which contains water and hydrogen, to a sulfur-containing fuel,will allow the sulfur to be removed from the fuel to the extentnecessary for use of the fuel as a hydrogen source for a mobile fuelcell power plant without poisoning the fuel cell power plant nickelreactant beds with sulfur. The sulfur compounds are removed from thefuel by means of a nickel reactant bed through which the fuel flowsprior to entering the fuel cell power plant's fuel processing section.The oxygenate, hydrogen-containing recycle, or water (steam) addition,also serves to control carbon deposition on the nickel reactant bedthereby extending its useful life and enhancing the sulfur removalcapabilities of the nickel reactant bed.

[0041] Since many changes and variations of the disclosed embodiment ofthe invention may be made without departing from the inventive concept,it is not intended to limit the invention otherwise than as required bythe appended claims.

What is claimed is:
 1. A method for desulfurizing a hydrocarbon fuelstream so as to convert the hydrocarbon fuel stream into a low sulfurcontent fuel, which low sulfur content fuel is suitable for use in afuel processing section in a fuel cell power plant, said methodcomprising the steps of: a) providing a nickel reactant desulfurizationstation which is operative to convert sulfur contained in organic sulfurcompounds contained in the fuel stream to nickel sulfide; b) introducinga hydrocarbon fuel stream which contains an oxygenate into said nickelreactant desulfurization station; and c) said oxygenate being present insaid fuel stream in an amount which is effective to provide an effluentfuel stream at an exit end of said nickel reactant station whicheffluent fuel stream contains no more than about 0.05 ppm sulfur.
 2. Themethod of claim 1 wherein the oxygenate is selected from the groupconsisting of water, alcohol, ether, and mixtures thereof.
 3. The methodof claim 2 wherein said oxygenate is present in amounts operable toprovide an operating life for the method which is at least about threetimes the operating life of a desulfurinzing method which does notinclude an oxygenate in the fuel stream.
 4. The method of claim 2wherein the oxygenate is selected from the group consisting of water,MTBE, ethanol, methanol, and mixtures thereof.
 5. A method fordesulfurizing a gasoline fuel stream so as to convert the gasoline fuelstream into a low sulfur content fuel, which low sulfur content fuel issuitable for use in a fuel processing section in a fuel cell powerplant, said method comprising the steps of: a) providing a nickelreactant desulfurization station which is operative to convert sulfurcontained in organic sulfur compounds contained in the fuel stream tonickel sulfide; b) introducing a gasoline fuel stream which contains anoxygenate into said nickel reactant desulfurization station; and c) saidoxygenate being present in said gasoline fuel stream in an amount whichis effective to provide an effluent gasoline fuel stream at an exit endof said nickel reactant station which effluent gasoline fuel streamcontains no more than about 0.05 ppm sulfur.
 6. The method of claim 5wherein the oxygenate is selected from the group consisting or water,alcohol, ether, and mixtures thereof.
 7. The method of claim 6 whereinthe oxygenate is selected from the group consisting of water, MTBE,ethanol, methanol, and mixtures thereof.
 8. A method for desulfurizing agasoline fuel stream so as to convert the gasoline fuel stream into alow sulfur content fuel, which low sulfur content fuel is suitable foruse in a fuel processing section of a fuel cell power plant, said methodcomprising the steps of: a) providing a nickel reactant desulfurizationstation which is operative to convert sulfur contained in organic sulfurcompounds contained in the fuel stream to nickel sulfide; b) introducinga gasoline fuel stream which contains an oxygenate into said nickelreactant desulfurization station; and c) said oxygenate being present insaid gasoline fuel stream in an amount which is effective to provide acontinuous gasoline fuel stream at an exit end of said nickel reactantstation which continuous gasoline fuel stream contains on average nomore than about 0.05 ppm sulfur.
 9. A method for desulfurizing agasoline fuel stream so as to convert the gasoline fuel stream into alow sulfur content fuel, which low sulfur content fuel is suitable foruse in a fuel processing section in a fuel cell power plant, said methodcomprising the steps of: a) providing a nickel reactant desulfurizationstation which is operative to convert sulfur contained in organic sulfurcompounds contained in the fuel stream to nickel sulfide; b) introducinga gasoline fuel stream which contains an oxygenate into said nickelreactant desulfurization station; and c) said oxygenate being convertedto isobutylene and methanol by said nickel catalyst in amounts which areeffective to inhibit carbon deposition in said nickel catalyst stationand provide a continuous gasoline fuel stream at an exit end of saidnickel reactant station which continuous gasoline fuel stream containsno more than about 0.05 ppm sulfur.
 10. A method for desulfurizing agasoline fuel stream so as to convert the gasoline fuel stream into alow sulfur content fuel, which low sulfur content fuel is suitable foruse in a fuel processing section in a fuel cell power plant, said methodcomprising the steps of: a) providing a nickel reactant desulfurizationstation which is operative to convert sulfur contained in organic sulfurcompounds contained in the fuel stream to nickel sulfide; b) introducinga gasoline fuel stream which contains an oxygenate into said nickelreactant desulfurization station, said oxygenate being present in saidgasoline fuel stream in an amount which is effective to provide a lowsulfur content gasoline fuel stream at an exit end of said nickelcatalyst station which low sulfur content gasoline fuel stream containsno more than about 0.05 ppm sulfur; and c) said oxygenate beingconverted to isobutylene and methanol by said nickel reactant duringsaid desulfurizing step, said low sulfur content gasoline fuel streambeing formed so long as said nickel reactant continues to convert theoxygenate.
 11. A method for desulfurizing a liquid gasoline fuel streamso as to convert the gasoline fuel stream into a low sulfur contentfuel, which low sulfur content fuel is suitable for use in a fuelprocessing section in a fuel cell power plant, said method comprisingthe steps of: a) providing a nickel reactant desulfurization stationwhich is operative to convert sulfur contained in organic sulfurcompounds contained in the fuel stream to nickel sulfide; b) maintainingsaid nickel reactant desulfurization station at a temperature in therange of about 300° F. to about 450° F.; c) introducing a liquidgasoline fuel stream which contains an oxygenate into said nickelreactant desulfurization station, said oxygenate being present in saidgasoline fuel stream in an amount which is effective to provide a lowsulfur content gasoline fuel stream at an exit end of said nickelreactant station which low sulfur content gasoline fuel stream containsno more than about 0.05 ppm sulfur; and d) said oxygenate beingconverted to isobutylene and methanol by said nickel reactant duringsaid desulfurizing step, said low sulfur content gasoline fuel streambeing formed so long as said nickel reactant continues to convert theoxygenate.
 12. A method for desulfurizing a liquid gasoline fuel streamso as to convert the gasoline fuel stream into a low sulfur contentfuel, which low sulfur content fuel is suitable for use in a fuelprocessing section in a fuel cell power plant, said method comprisingthe steps of: a) providing a nickel reactant desulfurization stationwhich is operative to convert sulfur contained in organic sulfurcompounds contained in the fuel stream to nickel sulfide; b) maintainingsaid nickel reactant desulfurization station at a temperature in therange of about 300° F. to about 450° F.; c) introducing a mixture ofabout 2% to about 5% water and a liquid gasoline fuel stream, whichmixture contains an oxygenate, into said nickel reactant desulfurizationstation, said oxygenate being present in said mixture in an amount whichis effective to provide a low sulfur content gasoline fuel stream at anexit end of said nickel reactant station, which low sulfur contentgasoline fuel stream contains no more than about 0.05 ppm sulfur; and d)said oxygenate being consumed by said nickel reactant during saiddesulfurizing step, said low sulfur content gasoline fuel stream beingformed so long as said nickel reactant continues to consume theoxygenate.
 13. The method of claim 12 wherein the water in said mixtureis derived by recirculating a portion of a selective oxidizer outputback to an inlet to said nickel reactant station.
 14. The method ofclaim 12 wherein the water in said mixture is the sole oxygenate in saidmixture.
 15. The method of claim 12 wherein the oxygenate includes analcohol present in said gasoline fuel stream.
 16. The method of claim 14wherein the alcohol is selected from the group consisting of methanol,ethanol, propanol, and mixtures thereof.
 17. The method of claim 12wherein said oxygenate is an ether.
 18. The method of claim 16 whereinsaid oxygenate is MTBE.
 19. The method of claim 12 wherein saidrecirculated portion of the selective oxidizer output is between 1% and10% of the total selective oxidizer output.
 20. A method fordesulfurizing a liquid gasoline fuel stream so as to convert thegasoline fuel stream into a low sulfur content fuel, which low sulfurcontent fuel is suitable for use in a fuel processing section in a fuelcell power plant, said method comprising the steps of: a) providing anickel reactant desulfurization station which is operative to convertsulfur contained in organic sulfur compounds contained in the fuelstream to nickel sulfide; b) maintaining said nickel reactantdesulfurization station at a temperature in the range of about 300° F.to about 450° F.; and c) introducing a mixture of a fuel cell selectiveoxidizer output recycle, which recycle contains hydrogen and water; anda liquid gasoline fuel, into said nickel reactant desulfurizationstation, said selective oxidizer output recycle being present in anamount which is effective to provide a low sulfur content gasoline fuelstream at an exit end of said nickel reactant station, which low sulfurcontent gasoline fuel stream contains no more than about 0.05 ppmsulfur.
 21. The method of claim 20 wherein said selective oxidizerrecycle comprises about 1% to about 10% of total selective oxidizeroutput.
 22. A method for desulfurizing a gaseous fuel stream so as toconvert the gaseous fuel stream into a low sulfur content fuel, whichlow sulfur content fuel is suitable for use in a fuel processing sectionin a fuel cell power plant, said method comprising the steps of: a)providing a nickel reactant desulfurization station which is operativeto convert sulfur contained in organic sulfur compounds contained in thefuel stream to nickel sulfide; b) introducing a gaseous fuel streamwhich contains a fuel cell selective oxidizer recycle mixture ofhydrogen and water into said nickel reactant desulfurization station;and c) said selective oxidizer recycle mixture being present in saidgaseous fuel stream in an amount which is effective to provide aneffluent gaseous fuel stream at an exit end of said nickel reactantstation which effluent gaseous fuel stream contains no more than about0.05 ppm sulfur.
 23. The method of claim 21 wherein the gaseous fuel isselected from the group consisting of methane, ethane, propane andbutane.
 24. The method of claim 21 wherein the desulfurization stationoperates in a temperature range of about 250° F. to about 450° F. 25.The method of claim 21 wherein said recirculated portion of theselective oxidizer output is between 1% and 10% of the total selectiveoxidizer output.